The present invention relates to a glass forming apparatus and method.
When a glass optical element, e.g., a glass lens, is formed, a forming method is generally employed in which a glass lump is ground and polished to form a desired final shape.
However, since this forming method comprises the complicated steps of grinding and polishing and requires much time for the steps, the cost is increased. Particularly, when an aspherical lens which has been greatly in demand in recent years is formed, the work becomes difficult because the steps are further complicated, and it is necessary for a skilled worker to finish the product in a final step.
On the other hand, a forming method which enables the omission of the final finishing step has been proposed.
For example, in a reheat press method, a preform (a glass preformed product) having a predetermined shape and mass is previously formed by grinding and is pressed with heating at a high temperature in a forming mold having high-precision surfaces. This method is capable of directly forming a glass lens having a desired shape.
However, in order to attain surface accuracy and dimensional precision which are required of a glass lens as a final product, it is necessary to sufficiently adjust the weight of the preform and to sufficiently finish the surfaces in the step of grinding and the like. This makes the work of forming the preform troublesome and increases the cost.
A direct press method of directly pressing molten glass without forming a preform has thus been proposed.
In this case, a glass flow is formed by the molten glass caused flow out or pushed out from an outflow orifice, is cut in a required amount by a cutting edge and is filled in a forming mold. In this case, the forming mold comprises a pair of left and right or upper and lower parts so that softened glass is directly held between the parts of the forming mold and pressed (Japanese Patent Laid-Open No. Hei 1-203234).
The direct press method generally uses a pair of cutting edges for cutting the glass flow. In this case, if the glass does not have proper viscosity (10.sup.3 to 10.sup.5 poise) a sound product can not be easily formed. Namely, for example, when the viscosity is high, a nonuniform cutting mark occurs at the front end and rear end of the cut glass flow and remains as a defect in a formed product. Conversely, when the viscosity is low, the glass flow cannot be easily cut, and defects such as bubbles, striae or the like easily occur.
Although various techniques of cutting and supplying a glass flow are proposed for solving the above problems, satisfactory results are not always obtained.
In addition, it is necessary for obtaining good forms by the direct press method to hold and press molten glass maintaining proper viscosity (10.sup.3 to 10.sup.5 poise) in the forming mold.
Although it is thus necessary to press the forming mold with high precision while controlling it in an appropriate high-temperature state, it is very difficult to control such a forming mold having a complicated mechanism in an appropriate high-temperature state. Further, since glass is excessively supplied to the forming mold, excess glass is extruded from the forming mold. A mechanism for removing the extruded glass is thus necessary, thereby further complicating the structure of the forming mold.
A forming method is thus proposed in which glass is filled in the cavity of a forming mold under pressure instead of pressing by moving the forming mold (Japanese Patent Laid-Open No. 49-81419 and 1-249630).
In this case, molten glass is filled in the cavity of the forming mold under pressure, and pressing is continued until the glass is solidified in order to prevent a shrink mark from occurring by shrinkage during solidification of the glass.
However, in this forming method, since the molten glass is supplied to the forming mold by sliding a plunger in a cylinder which is filled with the molten glass, the plunger and the cylinder are eroded by the glass, and abrasion or galling thus occurs thus making it difficult to stably supply the glass for a long time.
In the forming method disclosed in the Japanese Patent Laid-Open No. 49-81419, since a glass lump as a raw material must be inserted into a transfer chamber for each forming step, forming cannot be continuously performed, with reduction in efficiency.
In the forming method disclosed in the Japanese Laid-Open Application No. 1-249630, although forming can be continuously performed, the heat resistant structure and the temperature control of the overall forming apparatus are significantly complicated because the molten glass in a molten glass bath is supplied to the cylinder through a supply tube and injected by the plunger. In addition, since the raw material glass is held in a high-temperature melt state for a long time, the glass becomes heterogeneous in quality, and impurities are easily mixed therein.
In all the above methods, the complicated mechanism and steps increase the cost, and the quality of the formed product cannot be improved.